1. Field of the Invention
The present invention is related with a track jump controlling apparatus, which controls a track jump operation to move an optical pickup in the radial direction of a recording medium of disk shape.
2. Description of the Related Arts
An information reproducing apparatus for a recording medium of disk shape (hereinbelow, it is called as an optical disc), such as a LVD (Laser Vision Disk) and a CD (Compact Disk), is constituted as follows. Namely, the apparatus emits a laser light from a light source equipped in an optical pickup, to the optical disc, and obtains a RF (Radio Frequency) signal, a focus error signal, and a tracking error signal from the reflected light. By a control based on these obtained signals, special reproducing operations such as a still-picture reproduction, a slow reproduction, a double speed reproduction, etc. can be performed.
A track jump control is one of the indispensable techniques to perform the above-mentioned special reproducing operations. This track jump control is a control to move a beam spot, which is emitted from the optical pickup, to a desirable recording track (it is simply called as a "track" hereinbelow,) from a track where the present beam spot exists. More concretely, the track lump control is carried out by use of a kick pulse and a brake pulse, which have pulse widths, which correspond to the number of tracks to be jumped over, and predetermined crest values. These pulse widths and crest values are determined by the pitch between tracks of the optical disc, and the sensitivity of an actuator which drives the optical pickup. In case of 1 track jump, the pulse widths and crest values are set to draw an ideal velocity curve, in which the actuator can move the optical pickup in a minimum period of time to its adjacent track in a stable state. Here, the stable state means a state in which the velocity of the optical pickup i.e. the velocity of the beam spot, at the central line of the track of the jump destination becomes zero or becomes a value able to be adequately absorbed by a tracking servo-control as an external disturbance.
Nextly, the track jump control by an information recording and reproducing apparatus, will be explained with reference to FIG. 1. In this case, it is assumed that an information recording and reproducing apparatus has a tracking servo-loop to make the optical beam, for reading the record signal, track or follow a certain track on the optical disc, which is rotationally driven at a high speed, and that, in an initial state, the tracking servo-loop is in its closed state.
In order to simplify the explanation, it is also assumed that the actuator is a type without a spring system and ideally responds to an applied pulse.
For example, when a 1 track jump command is given, the micro computer for control, outputs a track jump command to change the tracking servo-loop to its open state. As shown in a time chart (a) of FIG. 1, the kick pulse KICK is applied to the actuator for driving the pickup. The kick pulse KICK has a polarity, which corresponds to the direction of the jump, a pulse width (=Tk) and a crest value (=current amount: IK) required for 1 track jump. As a result, the actuator starts the movement. Namely, if the kick pulse KICK is applied, the beam spot is moved in such a manner as to accelerate with a constant acceleration as shown in a time chart (b) of FIG. 1. When the tracking error signal obtained from the output signal of the pickup, arrives at the zero-crossing position as shown in a time chart (c) of FIG. 1, the brake pulse BRAKE is applied. The brake pulse BRAKE has a pulse width (=TB) and a crest value (=current amount: IB) required to apply the brake to the actuator, as shown in the time chart (a) of FIG. 1. Thereby, the beam spot decelerates with a constant deceleration as shown in the time chart (b) of FIG. 1. As shown in a chart (d) of FIG. 1, the tracking servo-loop is changed into its closed state at the center of the track i.e. at the time t2 (=the time when the time interval TB has elapsed from the time t1) when the velocity of the beam spot becomes zero. Then, the servo-control operation is performed, and the track jump is completed.
The above explanation has been made as for the case where the actual center of the optical disc and the rotation center of the spindle motor which rotationally drives the optical disc, are coincident to each other,namely, as for the ideal case where the eccentricity of the optical disc does not exists. However, an eccentricity certainly exists in an actual optical disc. For this reason, if the direction of the movement of the actuator, and the direction of the eccentricity are coincident to each other, the relative velocity between the movement of the beam spot, and the optical disc becomes lower. On the other hand, if these directions are opposite to each other, the relative velocity becomes higher. Therefore, only by simply applying to the actuator the kick pulse and brake pulse having the fixed pulse width and the fixed crest value defined beforehand on the supposition of the above mentioned ideal case, the kick operation is not always changed to the brake operation precisely at the central-axis between the tracks (i.e. the center of the grooves shown in the chart (d) of FIG. 1). That is to say, such a situation may happen that the tracks are crossed over excessively by jump, or a desired track is not crossed by jump, so that the jump to an intermediate track is performed, and in an extreme case, the jump to an opposite direction is performed.
Therefore, the information recording and reproducing apparatus may be constituted to detect the central axis between the tracks by use of the tracking error signal, which can be obtained in connection with the movement of the beam spot (refer to the time chart (c) of FIG. 1 ), to apply the kick pulse KICK to the actuator until the apparatus detects the zero-crossing point of the tracking error signal, and to apply the brake pulse BRAKE after the zero-crossing point is detected.
However, when the direction of the movement (fluctuation) of the track by the eccentricity of the optical disc, and the direction of the movement of the beam spot, are coincident to each other, in such an information recording and reproducing apparatus, the acceleration of the beam spot on the optical disc is cancelled at the time of acceleration as shown by a single chain line in a time chart (b) of FIG. 2. Thereby, as compared with the case where no eccentricity exists (as shown by a solid line in the time chart (b) of FIG. 2), the acceleration becomes less. On the contrary, the acceleration is added at the time of deceleration, so that , as compared with the case where no eccentricity exists, the acceleration becomes greater.
Similarly, when the direction of movement (fluctuation) of the track by the eccentricity of the optical disc, and the direction of the movement of the beam spot, are opposite to each other, the acceleration of the beam spot on the optical disc is added at the time of acceleration as shown by a dashed line in the time chart (b) of FIG. 2. Thereby, as compared with the case where no eccentricity exists, the acceleration becomes greater. On the contrary, the acceleration is cancelled at the time of deceleration, so that, as compared with the case where no eccentricity exists, the acceleration becomes less.
Apart from that, the kick pulse KICK is constituted such that the pulse width thereof is changed by the relative velocity, between the actuator and the optical disc, which changes due to the eccentricity. Therefore, if the brake pulse BRAKE, which has a fixed pulse width TB and a fixed crest value IB, is always applied at the center between the tracks (for example, the center of the grooves: the zero-crossing point of the tracking error) as shown in the charts (a) and (c) of FIG. 1 in case that the direction of the movement of the track by the eccentricity of the optical disc, and the direction of the movement of the beam spot, i.e. the actuator, are coincident to each other, as shown by the single chain line in the chart (b) of FIG. 2, the velocity is reduced too much. As the result, the velocity VJa to the opposite direction is remained at the jump destination. On the other hand, in case that these directions of the movement are opposite to each other, (as shown by the dashed line in the chart (b) of FIG. 2), the velocity cannot be adequately reduced so that the velocity VJc is remained at the jump destination. These residual velocities become external disturbances, from the viewpoint of the tracking servo-control, resulting in problems that the residual velocity cannot be fully absorbed by the tracking servo-control depending on the magnitude of the residual velocity, and that retracting to the target track after the jump operation, becomes unstable.